Motor speed control system

Abstract

A speed control system for a motor comprises a main motor circuit for supplying the motor current to the motor from an AC power supply via a main switch, a full-wave rectifier circuit and a semiconductor element with a control electrode, variable voltage setting means connected to the full-wave rectifier circuit, charge-discharge element connected to the variable voltage setting means and impressed with a voltage set by the variable voltage setting means, and a trigger voltage supply element conducted for releasing the charges charged in the charge-discharge element to the control electrode of the semiconductor element as a trigger voltage when the charge voltage of the charge-discharge element reaches a predetermined level. The speed control system further comprises means for detecting the electromotive force of the motor and automatically variable resistance means connected to the charge-discharge element and having its resistance value automatically varied in response to the electromotive force of the motor detected by the motor electromotive force detector means. The automatically variable resistance means controls the charging current to the charge-discharge element in accordance with the resistance value thereof. The conduction phase of the semiconductor element is retarded with an increase in motor electromotive force and advanced with a decrease in motor electromotiveforce.

I claim: 
     
       1. A speed control system for a motor comprising: a main motor circuit for supplying a motor current to the motor from an AC power supply through a main switch, a full-wave rectifier circuit and a semiconductor element with a control electrode; a variable voltage setting means connected to said full-wave rectifier circuit; a charge-discharge element impressed with a voltage set by said variable voltage setting means connected therewith; a trigger voltage supply element connected to said charge-discharge element, said trigger voltage supply element applying and releasing the charge in said charge-discharge element to said control electrode of said semiconductor element as a trigger voltage when the charge voltage of said charge-discharge element reaches a predetermined value; detector means for detecting the electromotive force of said motor, said detector means having a predetermined time constant; and automatically variable resistance means connected to said charge-discharge element and to said detector means, wherein the resistance value of said variable resistance means automatically varies in response to said motor electromotive force detected by said electromotive force detector means, said automatically variable resistance means controlling the charging current to said charge-discharge element in accordance with the resistance value thereof, the conduction phase of said semiconductor element being retarded with an increase in the motor electromotive force and advanced with a decrease in the electromotive force. 
     
     
       2. A speed control system for a motor according to claim 1, wherein said electromotive force detector means includes a parallel circuit of a resistor and a first capacitor connected in parallel to said motor, said automatically variable resistance means is a transistor, said parallel circuit being connected to base of said transistor, the collector-emitter circuit of said transistor being connected in parallel to said charge-discharge element thereby functioning as a bypass of the charge current to said charge-discharge element. 
     
     
       3. A speed control system for a motor according to claim 2, wherein said charge-discharge element is a second capacitor, said trigger voltage supply element being a bidirectional diode connected in parallel to said second capacitor, the emitter-collector circuit of said transistor being connected in series to a negative feedback resistor. 
     
     
       4. A speed control system for a motor according to claim 1, wherein said electromotive force detector means includes a parallel circuit of a constant voltage element and a first capacitor connected in parallel to said motor, said automatically variable resistance means is a transistor, said parallel circuit being connected to the base of said transistor, the collector-emitter circuit of said transistor being connected to said charge-discharge element. 
     
     
       5. A speed control system for a motor according to claim 4, wherein the collector-emitter circuit of said transistor being connected in series to said charge-discharge element, the base of said transistor being impressed with a forward bias in the form of the voltage set by said variable voltage setting means on the one hand and impressed with a reverse bias in the form of the electromotive force detected by said parallel circuit on the other hand. 
     
     
       6. A speed control system for a motor according to claim 5, wherein said charge-discharge element is a second capacitor, said trigger voltage supply element is bidirectional diode connected in parallel to said second capacitor, the emitter-collector circuit of said transistor being connected in series with a negative feedback resistor. 
     
     
       7. A speed control system for a motor according to claim 1, further comprising a switching element connected to said charge-discharge element and a first bias supply means connected to said full-wave rectifier circuit for applying a first bias for conducting said switching element, said first bias supply means supplying said first bias at a predetermined phase during each half cycle of the full-wave rectified current, said switching element being turned on while said first bias is being supplied thereto and releasing the charges from said charge-discharge element. 
     
     
       8. A speed control system for a motor according to claim 7, wherein said first bias supply means includes a parallel circuit of a constant voltage element and a first capacitor, the discharge current of said first capacitor being supplied as said first bias to said switching element. 
     
     
       9. A speed control system for a motor according to claim 8, wherein said switching element is a transistor, the base-emitter circuit of said transistor being connected in parallel to said parallel circuit, the discharge current of said first capacitor being supplied to the base of said transistor, the collector-emitter circuit of said transistor being connected in parallel to said charge-discharge element. 
     
     
       10. A speed control system for a motor acoording to claim 7, further comprising second bias supply means inserted between said semiconductor element with a control electrode and said switching element for supplying a second bias for conducting said switching element during the time when sawd semiconductor element is conducting. 
     
     
       11. A speed control system for a motor comprising: a main motor circuit for supplying a motor current to said motor from an AC power supply through a semiconductor element with a control electrode;   a controller of foot pedal type with a variable resistor for controlling the speed of said motor, the resistance of said variable resistor being changed by the press of said controller;   means for setting first and second trigger voltages for triggering said semiconductor element independently;   trigger voltage supply means for applying the voltage set by said first and second trigger voltage setting means as a trigger voltage to the control electrode of said semiconductor element;   trigger invalidating means adapted to be actuated for deactivating said second trigger means;   needle position detector means for detecting that the needle of the sewing machine has reached predetermined points;   first signal supply means for supplying a first signal for deactivating said trigger invalidating means when said main switch is closed;   first control means adapted to be actuated for invalidating said first signal;   second signal supply means for supplying a second signal for actuating said trigger invalidating means and a third signal for actuating said first control means when said controller is depressed;   second control means being actuated upon the lapse of a predetermined time after the depressing of said controller thereby invalidating said second and third signals;   third control means actuated by the depressing of said controller for invalidating said first and second signals, said third control means being deactivated in response to the operation of said second control means;   change-over means for switching between first and second modes;   means for detecting the reverse electromotive force of said motor at a predetermined time constant; and   automatically variable resistance means having its resistance value varied automatically in accordance with the difference between the voltage set by selected one of said first and second trigger voltage setting means and the electromotive force of said motor detected by said motor electromotive force detector means;   the voltage set by said first trigger voltage setting means being controlled in accordance with the resistance value of said variable resistor of said controller, said change-over means deactivating said second and third control means in said first mode, said change-over means deactivating said first trigger means in said second mode, said needle position detector means deactivating said first control means upon detection that said needle has reached said predetermined points, said automatically variable resistance means controlling the trigger voltage of said trigger voltage supply means in accordance with the resistance value thereof, the conduction phase of said semiconductor element being retarded with an increase in the electromotive force of said motor and advanced with a decrease in the electromotive force of said motor.   
     
     
       12. A speed control system for a motor according to claim 11, wherein said reverse electromotive force detector means includes a parallel circuit of a constant voltage element and a first capacitor connected in parallel to said motor, said trigger voltage supply means including a charge-discharge element impressed with a voltage set by said first and second trigger voltage setting means and a trigger voltage supply element connected to said charge-discharge element, said trigger voltage supply element being conducted and releasing the charges from said charge-discharge element when the charge voltage of said charge-discharge element reaches a predetermined value, said released charges being supplied as a trigger voltage to the control electrode of said semiconductor element, said automatically variable resistance means being connected in series with said charge-discharge element, the resistance value of said automatically variable resistance means varied in response to a voltage representing the difference between the voltage set by selected one of said first and second trigger voltage setting means and the detected electromotive force of said motor, thereby controlling the charging speed of said charge-discharge element. 
     
     
       13. A speed control system for a motor according to claim 12, wherein said charge-discharge element is a second capacitor, said trigger voltage supply element being a bidirectional diode connected in parallel to said second capacitor, said automatically variable resistance means being a transistor whose collector-emitter circuit is connected in series with said second capacitor, the base of said transistor being supplied with a forward bias in the form of the voltage set by said variable voltage setting means on the one hand and impressed with a reverse bias in the form of the electromotive force of the motor detected by said parallel circuit on the other hand.